A commercial gamma ray detector includes an array of scintillator crystals coupled to a transparent light guide, which distributes scintillation light over an array of photomultiplier tubes (PMTs) arranged over the transparent light guide. Signals from the PMTs in a same area are generally summed in the analog domain, and then timing is measured based on the leading edge of the summed signal, or event.
A Time-to-Digital-Converter (TDC) is often used to measure timing in a gamma ray detector. A TDC accurately converts the realization of an event into a number than can be related to the time the event occurred. Various methods exist to perform this task. Amongst others, counting a large number of very fast logic transitions between coarse clock cycles has been used to perform this task. In some cases, it may be desirable to indicate the occurrence of a series of events known to be generated sequentially. For instance, time marks a rising signal takes to reach a pre-determined set of threshold values can be very useful information.
Time-to-digital converters (TDCs) have also been implemented with a variety of architectures. A first conventional architecture is a classic delay chain having a single chain of identical delay elements connected in series. The classic delay chain also includes a set of single bit memory elements, each connected to an output of one of the delay elements. A start signal is supplied to the input of the chain of delay elements to indicate a beginning of the time period to be measured. The start signal propagates through the chain of delay elements. The end of the time period to be measured is indicated by a stop signal that is simultaneously provided to the clock inputs of all of the memory elements in order to capture the position of the propagated start signal within the chain of delay elements. The captured position is then thermometer-decoded to compute the delay between the start and stop signals, and this delay is used to compute the length of the time period to be measured as a multiple of the delay imparted by each of the delay elements.
Therefore, the resolution of the classic delay chain is limited to the time-delay of each delay element in the delay chain. For example, if each delay element in the chain imparts a delay of “tu”, then the resolution of the classic delay chain is “tu”. As such, in a physical implementation of the classic delay chain, such as in a semiconductor device, the minimum value of tu is limited by the physical properties of the semiconductor. As sampling is performed at the same point in time for each delay element in the classic delay chain, the physical limitations on the delay tu give rise to the limits of measurement resolution.
Another conventional delay chain is the Vernier delay chain. As in the classic delay chain, the Vernier delay chain includes a chain of identical delay elements connected in series and a set of single bit memory elements, each connected to the output of one of the delay elements. However, the Vernier delay chain also includes a second delay chain of identical delay elements connected in series. The output of each of the delay elements in the second delay chain is connected to a clock input of one of the memory elements. Further, the delays elements in the first delay chain each impart a delay of tu, and the delay elements of the second delay chain each impart a delay of tc, where tc<tu.
In operation, the start signal is supplied to the first delay chain of the Vernier delay chain, and the stop signal is supplied to the second delay chain. As the delay imparted by the elements of the second delay chain is less than that of the elements of the first delay chain, the stop signal will eventually overtake the start signal. When the stop signal overtakes the start signal, the propagation of the start signal in the first delay chain is captured by the memory elements and thermometer-decoded to determine the time interval between the start and stop signals. The time period to be measured is then calculated as a multiple of the difference between the delays of the first delay chain and the delays of the second delay chain, or tu−tc.
As with the classic delay chain, the delays in the Vernier delay chain are limited by the physical properties of the semiconductor device on which it is implemented. Therefore, there is a minimum delay difference (tu−tc) (i.e. resolution) that can be achieved using the Vernier delay chain. Thus, it is difficult to make precise time period measurements using the Vernier delay chain.
Accordingly, a need exists for an apparatus and associated methodology that improves upon the limitations of the classic and Vernier delay chains, and that achieves improved accuracy and resolution.